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 A232636 The second largest value of permanent of (0,1) square matrices of order n with row and column sums equal to 3. 0
 34012224, 53747712, 131010048, 204073344, 322486272, 786060288, 1224440064, 1934917632, 4716361728, 7346640384, 11609505792, 28298170368, 44079842304, 69657034752, 169789022208, 264479053824, 417942208512 (list; graph; refs; listen; history; text; internal format)
 OFFSET 30,1 COMMENTS The interval (a(n), A232553(n)) contains no permanents of the matrices under consideration. For n=3 and n=4, the permanent takes only one value: 6 and 9 respectively. Our method (see Shevelev link) allows us to find a simple regularity of the sequence only beginning with n=30 (see formula). However, several values for 5<=n<30 are known: a(5)=13, a(6)=20, a(7)=32, a(8)=78, a(9)=120, a(12)=729, a(15)=4374, a(18)=26244, a(21)=157464, a(24)=944784, a(27)=5668704 (Bolshakov) and a(28)=8957952. We conjecture that formula below also holds for other values. In cases n==0 (nod 3), n>=6, and n==2 (mod 3), at least, for n=8 and n>=32, the second largest value of permanent attains on subset of symmetric matrices with the main diagonal of 1's, and does not attain, if n==1 (mod 3), at least, for n=7 and n>=28. All terms for n>30 not divisible by 3 are conjectural since our method is based on the following very plausible but yet not proved conjecture with confirms in many cases and without counterexample since 1992 (when it was posed, see ref.  in Shevelev link). Let L_n be the set of considered n X n (0,1)-matrices with all row and column sums equal to 3. Let S_n be the subset of completely indecomposable such matrices, i.e., not containing submatrices from L_m (m=12, n==0 (mod 3) (Bolshakov); a(n) = 8/9 * 6^((n-1)/3), if n>=28, n== 1 (mod 3); a(n) = 13/6 * 6^((n-2)/3), if n>=32, n==2 (mod 3). Conjectures from Colin Barker, May 27 2016: (Start) a(n) = 6*a(n-3) for n>32. G.f.: 419904*x^30*(81+128*x+312*x^2) / (1-6*x^3). (End) CROSSREFS Cf. A232553, A176211, A176212, A185177, A185178, A185179. Sequence in context: A143511 A178106 A205943 * A172687 A332388 A271769 Adjacent sequences:  A232633 A232634 A232635 * A232637 A232638 A232639 KEYWORD nonn AUTHOR Vladimir Shevelev, Nov 27 2013 EXTENSIONS Edited by M. F. Hasler, Nov 30 2013 STATUS approved

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Last modified July 9 14:56 EDT 2020. Contains 335543 sequences. (Running on oeis4.)